The present invention relates to plasma etching apparatus and method and, more particularly, to apparatus and method for providing highly uniform etching of a semiconductor workpiece while permitting enhanced optical access to the workpiece for process monitoring and control.
Plasma etching has become important in the manufacture of semiconductor devices such as integrated circuits. Plasma etching is the selective removal of material from a workpiece by reactive chemical species generated within a glow discharge ambient. In many applications, plasma etching is superior to wet etching with regard to ease of operation, cleanliness and process control.
A typical plasma etching process involves loading one or more workpieces (typically semiconductor wafers) into a chamber, sealing the chamber and evacuating it to a low pressure in the range from about 0.2 Torr to about 5 Torr. A selected reagent gas is introduced into the chamber and is ionized by an applied radio frequency (RF) field. The resulting glow discharge contains equal numbers of positive and negative ions and a population of free radicals which are electrically neutral, highly reactive chemical species capable of etching semiconductor materials. Upon completion of the etching cycle, the reagent gas flow is terminated and the RF generator turned off. After the chamber is exhausted of reaction products, it is back-filled with inert gas, such as nitrogen. The chamber is then opened, and the workpieces are removed.
In reactive ion etching, termed RIE, plasma etching is enhanced by attracting ions from the plasma onto the workpiece. RIE etching is typically conducted at a pressure in the range between 2.times.10.sup.-4 Torr and 0.2 Torr. Through one of a variety of postulated mechanisms, the colliding ions enhance the etch rate and impart direction to the etching process. Thus while unassisted plasma etching is isotropic, reactive ion etching proceeds primarily in the direction of the impinging ions. The advantage of RIE is that it etches relatively straight walls of uniform cross section as distinguished from the undercut walls typically produced by isotropic plasma etching.
One practical difficulty with reactive ion etching is non-uniform etching both on a macroscale from one end of the workpiece to the other and on a microscale in the vicinity of fine features. Efforts at minimizing macroscale nonuniformity have concentrated primarily on generating a uniform plasma and a uniform gas flow in the vicinity of the workpiece. The problem of nonuniformity on a microscale has not heretofore been adequately addressed.
Increasingly sophisticated techniques have been proposed for optically inspecting etched workpieces and even optically monitoring the workpieces during the etching process. However the practical utility of such optical monitoring processes has been limited by the presence of structures heretofore needed to enhance etching uniformity but which have the disadvantage of blocking optical access to the workpiece.
For example, a typical reactive ion etching apparatus for commercial use is described in U.S. Pat. No. 4,595,484 to N. J. Giammarco, et al. The apparatus comprises a process chamber, a cathode upon which the workpiece is mounted, and a three-plate anode arranged over the cathode. The anode plates are perforated to enhance uniformity of gas flow, plasma and etching on the underlying workpiece. While this anode arrangement is directed to improving uniformity on a macroscale, it does so at the cost of blocking optical access to the workpiece. Three layers of perforated metal over the workpiece foreclose optical techniques for monitoring and controlling the etching process.